Today's optical networks are mostly ring-based but are moving toward mesh-based. A mesh architecture has several advantages over a ring architecture, such as more efficient bandwidth utilization, more diverse protection, and less constrained network growth. At the mesh nodes, one would like to be able to route wavelengths arbitrarily, using a wavelength-selective cross connect. The number of fibers entering the node determines its degree.
Wavelength-selective cross connects may be built out of wavelength-selective switches (WSSs). There are two main types of WSSs: transmissive and reflective. In a transmissive WSS, the input is directed in a one-way fashion to one of the K outputs, and the input is clearly distinct from the outputs. An example is the planar lightwave circuit (PLC) 1×9 WSS demonstrated in [1]. (Note, a reference number in a bracket [ ] refers to a publication listed in the attached Reference list.) In a reflective WSS, the input is reflected back by a steering mirror, being directed to one of the K outputs; and the input is not distinct from the outputs. An example is the 1×4 WSS demonstrated in [2], which used a bulk grating and micro-electro mechanical systems (MEMS) tilt mirrors. Another example is one using a vertical stack[3] or horizontal arrangement[4] of PLCs and MEMS tilt mirrors.
While the designs of such reflective WSS based mesh nodes have proven to be highly flexible, their complexity increases significantly with increasing numbers of ports. Moreover, since in reflective WSS based mesh node designs the stray connections become almost unmanageable for nodes with degree higher than 6, such designs are limited to degree 6 nodes. Thus there is continuing need to simplify the design WSS-based mesh nodes.